1. Field of the Invention
This invention relates to a method for forming a thin-film in a desired pattern, and particularly to a method for forming a multilayer interconnection pattern in a process for manufacturing LSIs, in particular MOSFETs.
2. Description of the Prior Art
In forming interconnections and leads in LSIs in accordance with the prior art, a thin-film of an electrically conductive material, such as Al and Al-Si, is first formed by evaporation, sputtering, etc. and then after patterning, it is subjected to heat treatment at the temperature ranging from 400.degree. C. to 500.degree. C. However, it is known that ridges called whiskers or hillocks are formed at the surface during this heat treatment thereby destroying smoothness in the surface. Such whiskers and hillocks could cause disconnections in interconnection lines due to electromigration, and, in particular, they could cause an electrical short between a first lead layer and a second lead layer in a multilayer interconnection structure, thereby making the multilayer interconnection structure defective.
Japanese Patent Laid-open Publication, No. 57-183053, etc. discloses a method for preventing the formation of whiskers in a main surface of Al by implanting impurities, such as P, As and Ar. In the publication, it is also described that production of whiskers is a phenomenon peculiar to pure Al and no such phenomenon takes place for alloy materials, such as Al-Si. On the other hand, with respect to hillocks, a paper presented by T. J. Faith, in J. Appl. Phys., Vol. 52, No. 7, July 1981, includes a description which indicates that ridges in the form of hillocks may be formed in the surface of pure Al. As described in the LSI DATA HANDBOOK, Science Forum Co., pp. 316-323, one approach to prevent the formation of such hillocks is to use an electrically conductive material with an impurity, such as Si, Cu and Mg; however, for Al-Si, for example, the effects are not sufficient, and, for Al-Si-Cu, there are problems including difficulty in etching, Cu residues and contamination inside of the sputtering device by Cu. Another approach is to introduce O, but this cannot be easily put into practice because of an increase in resistance and difficulty in controlling the introduction of O.
Moreover, in forming interconnection patterns in LSIs, chemical wet etching has also been used to etch an Al pattern into a desired pattern. Although wet etching of Al is simple, it can no longer be applied when a desired line width of Al becomes 3 microns or less. One of the reasons for this is the formation of hillocks. Thus, in forming a fine metal pattern, wet etching cannot be applied.